The present invention is directed in general to optical arrangements for projecting light output of lasers. The invention is directed in particular to an optical arrangement for projecting a uniform beam from the output of a multimode diode-laser.
Semiconductor diode-lasers are finding application as sources of therapeutic treatment radiation in many types of scientific and medical apparatus. They have advantages in that they are small, efficient, reliable, and can have emission wavelengths selected within a wide range of wavelengths by appropriate selection of the composition of active semiconductor materials of the diode-lasers.
Diode-lasers used in medical apparatus in particular are multimode diode-lasers. The multimode operation results from providing a relatively wide active-region in the diode-laser to increase the amount of power generated by the laser. The wide active-region gives rise to multiple transverse lasing modes. A disadvantage of a multimode diode-laser is that light output from an emitting aperture of the laser which may be as much as 100 times as wide as it is high provides a highly elliptical output beam. The light diverges relatively narrowly, for example at about 10 degrees in the width direction of the aperture (the slow axis), and more widely, for example at about 35 degrees in the height direction (fast axis) of the aperture.
Light output from a multimode diode-laser is often transported to a point of use by an optical fiber. The diode-laser-light is collected into optical fiber by a lens, for example, a cylindrical lens disposed between the diode-laser and the optical fiber and aligned in the slow axis. The combination of the lens and the optical fiber provides for a certain degree of xe2x80x9ccircularizationxe2x80x9d of the diode-laser-light output. Typically, light is output from the optical fiber as a beam having a generally circular cross-section and a numerical aperture (divergence) which is about the average of the numerical aperture of the diode-laser output in the fast and slow axes. A significant problem, however, is that the intensity distribution of light output of the optical fiber in the near field is uneven due to the multiple modes propagated in the optical fiber. This uneven intensity distribution as well as the numerical aperture of the optical fiber output can fluctuate due to temporal changes in the light output of the diode-laser or changes in orientation or bending of the optical fiber.
The present invention is directed to a method and apparatus for delivering a laser beam from a laser via an optical fiber with the laser beam having a substantially uniform near-field intensity distribution on delivery from the optical fiber, for example, with variation less than about xc2x110 percent of a nominal average. The uniform laser beam may be delivered to an optical apparatus for use therein. In one aspect, apparatus in accordance with the present invention includes a laser delivering a beam of laser-light in one or more modes and first and second multimode optical fibers.
An optical arrangement receives the laser-light from the laser and directs the laser-light into the first optical fiber at an entrance end thereof. The first optical fiber is configured such that the laser-light exits the first optical fiber at an exit end thereof in a second number of modes, the second number of modes being greater than the first number of modes. An optical system receives the laser-light exiting the first optical fiber and directs the laser-light into the second optical fiber at an entrance end thereof. The second optical fiber is configured such that the laser-light exits the second optical fiber at an exit end thereof as an output beam in a third number of modes, the third number of modes being greater than the second number of modes.
Increasing the number of modes in the laser-light by transporting the laser-light through the second optical fiber combined with mixing of the modes within the second optical fiber provides that the near field uniformity of intensity of the output beam at the exit end of the second optical fiber is better than the near-field uniformity of intensity of the laser-light beam emitted at the exit end of the first optical fiber.
In one embodiment of the present invention, a portion of the second optical fiber is retained in the form of one or more bends. Selecting an appropriate number and radius of curvature of the one more bends provides that the near-field intensity distribution across the output beam is substantially constant or uniform.
In one particular example of apparatus in accordance with the present invention, the apparatus is directed to providing a beam of substantially uniform intensity distribution and variable size at a focal plane of an ophthalmic slit-lamp microscope for use in treatment of age-related macular degeneration. The laser is a multimode diode-laser emitting at a wavelength of about 689 nanometers (nm). The first optical fiber has a core diameter of 100.0 xcexcm and length of about 1.0 meters (m). The second optical fiber has a core diameter of 200.0 xcexcm and a length of about 3 m. A portion of the second optical fiber is retained in a loop or 360xc2x0 bend having a diameter of about 30 millimeters (mm). The output beam at the end of the second optical fiber has a uniformity of intensity better than xc2x110% over a central 90% of the beam. A second optical system receives the output beam and provides a magnified image of the end of the optical fiber at a focal plane of the slit-lamp microscope. The image, of course, has essentially the same uniformity of illumination intensity as the output beam at the end of the optical fiber. The second optical system is a zoom optical system arranged such that the image can be selectively adjusted in diameter between about 0.4 mm and 5.0 mm.